Improved group iii nitride substrate, method of making, and method of use

ABSTRACT

Embodiments of the present disclosure include techniques related to techniques for processing materials for manufacture of group-III metal nitride and gallium based substrates. More specifically, embodiments of the disclosure include techniques for growing large area substrates using a combination of processing techniques. Merely by way of example, the disclosure can be applied to growing crystals of GaN, AlN, InN, InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk or patterned substrates. Such bulk or patterned substrates can be used for a variety of applications including optoelectronic and electronic devices, lasers, light emitting diodes, solar cells, photo electrochemical water splitting and hydrogen generation, photodetectors, integrated circuits, and transistors, and others.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit to U.S. provisional application No.62/975,078, filed Feb. 11, 2020, which is incorporated by referenceherein.

BACKGROUND Field

This disclosure relates generally to techniques for processing materialsfor manufacture of gallium-containing nitride substrates and utilizationof these substrates in optoelectronic and electronic devices. Morespecifically, embodiments of the disclosure include techniques forgrowing large area substrates using a combination of processingtechniques.

Description of the Related Art

Gallium nitride (GaN) based optoelectronic and electronic devices are oftremendous commercial importance. The quality and reliability of thesedevices, however, is compromised by high defect levels, particularlythreading dislocations, grain boundaries, and strain in semiconductorlayers of the devices. Threading dislocations can arise from latticemismatch of GaN based semiconductor layers to a non-GaN substrate suchas sapphire or silicon carbide. Grain boundaries can arise from thecoalescence fronts of epitaxially-overgrown layers. Additional defectscan arise from thermal expansion mismatch, impurities, and tiltboundaries, depending on the details of the growth of the layers.

The presence of defects has deleterious effects on epitaxially-grownlayers. Such effects include compromising electronic device performance.To overcome these defects, techniques have been proposed that requirecomplex, tedious fabrication processes to reduce the concentrationand/or impact of the defects. While a substantial number of conventionalgrowth methods for gallium nitride crystals have been proposed,limitations still exist. That is, conventional methods still meritimprovement to be cost effective and efficient.

Progress has been made in the growth of large-area gallium nitridecrystals with considerably lower defect levels than heteroepitaxial GaNlayers. However, most techniques for growth of large-area GaN substratesinvolve GaN deposition on a non-GaN substrate, such as sapphire or GaAs.This approach generally gives rise to threading dislocations at averageconcentrations of 10⁵-10⁷ cm⁻² over the surface of thick boules, as wellas significant bow, stress, and strain. Reduced concentrations ofthreading dislocations are desirable for a number of applications. Bow,stress, and strain can cause low yields when slicing the boules intowafers, make the wafers susceptible to cracking during down-streamprocessing, and may also negatively impact device reliability andlifetime. Another consequence of the bow, stress, and strain is that,during growth in m-plane and semipolar directions, even bynear-equilibrium techniques such as ammonothermal growth, significantconcentrations of stacking faults may be generated. In addition, thequality of c-plane growth may be unsatisfactory, due to formation ofcracks, multiple crystallographic domains, and the like. Capability tomanufacture substrates larger than 2 inches is currently very limited,as is capability to produce large-area GaN substrates with a nonpolar orsemipolar crystallographic orientation. Most large area substrates aremanufactured by vapor-phase methods, such as hydride vapor phase epitaxy(HVPE), which are relatively expensive. A less-expensive method isdesired, while also achieving large area and low threading dislocationdensities as quickly as possible.

Ammonothermal crystal growth has a number of advantages over HVPE as ameans for manufacturing GaN boules. However, the performance ofammonothermal GaN crystal growth processing may be significantlydependent on the size and quality of seed crystals. Seed crystalsfabricated by HVPE may suffer from many of the limitations describedabove, and large area ammonothermally-grown crystals are not widelyavailable.

Lateral epitaxial overgrowth (LEO) is a method that has been widelyapplied to improvement in the crystallographic quality of films grown byvapor-phase methods. Several authors have disclosed methods forperforming lateral growth from sidewalls of thin-film GaN layers onnon-GaN substrates. However, to the best of our knowledge, analogousmethods have not yet been disclosed for bulk growth of GaN, includingammonothermal growth, and the methods that have been disclosed forsidewall LEO of thin-film GaN are impractical for bulk GaN.

Due to at least the issues described above, there is a need forsubstrates that have a lower defect density and are formed by techniquesthat improve the crystal growth process. Also, from the above, it isseen that techniques for improving crystal growth are highly desirable.

SUMMARY

Embodiments of the present disclosure include a free-standing group IIImetal nitride crystal. The free-standing crystal comprises a wurtzitecrystal structure, a first surface having a maximum dimension greaterthan 40 millimeters in a first direction, an average concentration ofstacking faults below 10³ cm⁻¹; an average concentration of threadingdislocations between 10¹ cm⁻² and 10⁶ cm⁻², wherein the averageconcentration of threading dislocations on the first surface variesperiodically by at least a factor of two in the first direction, theperiod of the variation in the first direction being between 5micrometers and 20 millimeters, and a miscut angle that varies by 0.1degree or less in the central 80% of the first surface of the crystalalong the first direction and by 0.1 degree or less in the central 80%of the first surface of the crystal along a second direction orthogonalto the first direction. The first surface comprises a plurality of firstregions, each of the plurality of first regions having alocally-approximately-linear array of threading dislocations with aconcentration between 5 cm⁻¹ and 10⁵ cm⁻¹, the first surface furthercomprises a plurality of second regions, each of the plurality of secondregions being disposed between an adjacent pair of the plurality offirst regions and having a concentration of threading dislocations below10⁵ cm⁻² and a concentration of stacking faults below 10³ cm⁻¹, and thefirst surface further comprises a plurality of third regions, each ofthe plurality of third regions being disposed within one of theplurality of second regions or between an adjacent pair of second andhaving a minimum dimension between 10 micrometers and 500 micrometersand threading dislocations with a concentration between 10³ cm⁻² and 10⁸cm⁻².

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlyexemplary embodiments and are therefore not to be considered limiting ofits scope, may admit to other equally effective embodiments.

FIGS. 1A, 1B, and 1C are simplified diagrams illustrating differentstages of a method of forming a patterned photoresist layer on a seedcrystal or a substrate according to an embodiment of the presentdisclosure.

FIGS. 1D and 1E are simplified diagrams illustrating a method of forminga patterned mask layer on a seed crystal or a substrate, according to anembodiment of the present disclosure.

FIGS. 1F, 1G, 1H, 1I, and 1J are top views of arrangements of openingsin a patterned mask layer on a seed crystal or a substrate according toan embodiment of the present disclosure.

FIGS. 1K and 1L are top views of arrangements of openings in a patternedmask layer on a seed crystal or a substrate according to an embodimentof the present disclosure.

FIGS. 1M and 1N are simplified diagrams illustrating different stages ofa method of forming a patterned photoresist layer on a seed crystal or asubstrate according to an alternate embodiment of the presentdisclosure.

FIGS. 10 and 1P are simplified diagrams illustrating a method of forminga patterned mask layer on a seed crystal or a substrate, according to analternate embodiment of the present disclosure.

FIG. 1Q is a simplified diagram illustrating a method of formingpatterned trenches within a seed crystal or a substrate, according to anembodiment of the present disclosure.

FIGS. 1R, 1S, and 1T are simplified diagrams illustrating an alternativemethod of forming patterned trenches within a seed crystal or asubstrate, according to an embodiment of the present invention.

FIGS. 2A, 2B, and 2C are simplified diagrams illustrating an epitaxiallateral overgrowth process for forming a large area group III metalnitride crystal, according to an embodiment of the present disclosure.

FIGS. 3A, 3B, and 3C are simplified diagrams illustrating an improvedepitaxial lateral overgrowth process for forming a large area group IIImetal nitride crystal, according to an embodiment of the presentdisclosure.

FIGS. 3D, 3E, and 3F are simplified diagrams illustrating an improvedepitaxial lateral overgrowth process for forming a large area group IIImetal nitride crystal, according to an embodiment of the presentdisclosure.

FIGS. 4A and 4B are simplified diagrams illustrating a method of forminga free-standing ammonothermal group III metal nitride boule andfree-standing ammonothermal group III metal nitride wafers.

FIGS. 5A-5E are simplified diagrams illustrating threading dislocationpatterns and regions on a free-standing ammonothermal group III metalnitride boule or wafer according to an embodiment of the presentdisclosure.

FIGS. 6A, 6B, 7A, and 7B are cross-sectional diagrams illustratingmethods and resulting optical, opto-electronic, or electronic devicesaccording to embodiments of the present disclosure.

FIG. 8 is a top view (plan view) of a free-standing laterally-grown GaNboule or wafer formed by ammonothermal lateral epitaxial growth using amask layer having openings arranged in a two-dimensional square array.

FIGS. 9A, 9B, and 9C are top views of a device structure, for example,of LEDs, according to embodiments of the present disclosure.

FIG. 10 is an optical micrograph of a polished cross section of a trenchin a c-plane GaN substrate that has been prepared according to anembodiment of the present disclosure.

FIGS. 11A and 11B are optical micrographs of a cross section of ac-plane ammonothermal GaN layer that has formed according to anembodiment of the present disclosure, together with a close-up imagefrom the same cross section.

FIGS. 12A and 12B are plan-view optical micrographs of c-planeammonothermal GaN layers that have been subjected to defect-selectiveetching, showing a low concentration of etch pits in the window regionabove slit-shaped mask openings oriented along a <10-10> direction and alinear array of etch pits (threading dislocations) at coalescence frontsformed approximately midway between two window regions, according to twoembodiments of the present disclosure.

FIG. 13 is a summary of x-ray diffraction measurements comparing themiscut variation across a 50 mm wafer fabricated according to oneembodiment of the present disclosure with that of acommercially-available 50 mm wafer.

FIG. 14 is a summary of x-ray rocking-curve measurements comparing thefull-width-at-half-maximum values of two reflections from a 50 mm waferfabricated according to one embodiment of the present disclosure withthat of a commercially-available 50 mm wafer.

FIG. 15 is an optical micrograph of a laser-cut cross section of atrench in a c-plane GaN substrate that has been prepared according to anembodiment of the present disclosure.

FIG. 16 is a plan-view optical micrograph of a c-plane ammonothermal GaNlayer that has been subjected to defect-selective etching, showing a lowconcentration of etch pits in the window region above slit-shaped maskopenings oriented along a <10-10> direction and a linear array of etchpits (threading dislocations) at coalescence fronts formed approximatelymidway between two window regions, according to an embodiment of thepresent disclosure.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements and features of oneembodiment may be beneficially incorporated in other embodiments withoutfurther recitation.

DETAILED DESCRIPTION

According to the present disclosure, techniques related to techniquesfor processing materials for manufacture of group-III metal nitride andgallium based substrates are provided. More specifically, embodiments ofthe disclosure include techniques for growing large area substratesusing a combination of processing techniques. Merely by way of example,the disclosure can be applied to growing crystals of GaN, AlN, InN,InGaN, AlGaN, and AlInGaN, and others for manufacture of bulk orpatterned substrates. Such bulk or patterned substrates can be used fora variety of applications including optoelectronic devices, lasers,light emitting diodes, solar cells, photo electrochemical watersplitting and hydrogen generation, photodetectors, integrated circuits,and transistors, and others.

Threading dislocations in GaN are known to act as strong non-radiativerecombination centers which can severely limit the efficiency ofGaN-based LEDs and laser diodes. Non-radiative recombination generateslocal heating which may lead to faster device degradation (Cao et al.,Microelectronics Reliability, 2003, 43(12), 1987-1991). In high-powerapplications, GaN-based devices suffer from decreased efficiency withincreasing current density, known as droop. There is evidence suggestinga correlation between dislocation density and the magnitude of droop inLEDs (Schubert et al., Applied Physics Letters, 2007, 91(23), 231114).For GaN-based laser diodes there is a well-documented negativecorrelation between dislocation density and mean time to failure (MTTF)(Tom iya et al., IEEE Journal of Selected Topics in Quantum Electronics,2004, 10(6), 1277-1286), which appears to be due to impurity diffusionalong the dislocations (Orita et al., IEEE International ReliabilityPhysics Symposium Proceedings, 2009, 736-740). For electronic devices,dislocations have been shown to markedly increase the leakage current(Kaun et al., Applied Physics Express, 2011, 4(2), 024101) and reducethe device lifetime (Tapajna et al., Applied Physics Letters, 2011,99(22), 223501-223503) in HEMT structures. One of the primary advantagesof using bulk GaN as a substrate material for epitaxial thin film growthis a large reduction in the concentration of threading dislocations inthe film. Therefore, the dislocation density in the bulk GaN substratewill have a significant impact on the device efficiency and thereliability.

As noted above, lateral epitaxial overgrowth (LEO) is a method that hasbeen widely applied to improvement in the crystallographic quality offilms grown by vapor-phase methods. For example, methods whereby GaNlayers were nucleated on a sapphire substrate, a SiO₂ mask with aperiodic array of openings was deposited on the GaN layer, and then GaNwas grown by metalorganic chemical vapor deposition (MOCVD) through theopenings in the SiO₂ mask layer, grew laterally over the mask, andcoalesced. The dislocation density in the areas above the openings inthe mask were very high, similar to the layer below the mask, but thedislocation density in the laterally-overgrown regions was orders ofmagnitude less. This method is attractive because it can be applied tolarge area substrates, significantly reducing their dislocation density.Similar methods, with variations, have been applied by a number ofgroups to vapor-phase growth of GaN layers. These methods are variouslyreferred to as LEO, epitaxial lateral overgrowth (ELO or ELOG),selective area growth (SAG), and dislocation elimination by epitaxialgrowth with inverse pyramidal pits (DEEP), or the like. In the case ofessentially all variations of this method, it is believed that a thinheteroepitaxial GaN layer is grown on a non-GaN substrate, a patternedmask is deposited on the GaN layer, and growth is re-initiated in aone-dimensional or two-dimensional array of openings in the mask. Theperiod or pitch of the growth locations defined by the openings in themask is typically between 2 and 100 micrometers, typically between about5 and 20 micrometers. The individual GaN crystallites or regions growand then coalesce. Epitaxial growth may then be continued on top of thecoalesced GaN material to produce a thick film or “ingot.” A relativelythick GaN layer may be deposited on the coalesced GaN material by HVPE.The LEO process is capable of large reductions in the concentration ofdislocations, particularly in the regions above the mask, typically tolevels of about 10⁵-10⁷ cm⁻². However, very often the laterally-grownwings of the formed LEO layer are crystallographically tilted from theunderlying substrate (“wing tilt”), by as much as several degrees, whichmay be acceptable for a thin-film process but may not be acceptable fora bulk crystal growth process, as it may give rise to stresses andcracking as well as unacceptable variation in surface crystallographicorientation.

Several factors limit the capability of the LEO method, asconventionally applied, to reduce the average dislocation density belowabout 10⁵ to 10⁷ cm⁻², or to reduce the miscut variation across a 50 or100 mm wafer to below about 0.1 degree. First, the pitch of the patternof openings formed in the mask layer tends to be modest, but largerpitches may be desirable for certain applications. Second, c-plane LEOgrowth is generally performed in the (0001), or Ga-face direction, whichcreates at least two limitations. One limitation is that M-directiongrowth rates tend to be lower than those of (0001)-direction growthrates and semipolar (10-11) facets often form, with the consequence thatthe overall crystal diameter decreases with increasing thickness andmaking coalescence of large-pitch patterns difficult. In addition,another limitation is that growth in the (0001) direction tends toexclude oxygen, in contrast to growth in other crystallographicdirections. As a consequence, there may be a significant latticemismatch between a (0001)-grown HVPE crystal used as a seed and thecrystal grown upon it by another technique. In addition, if semipolarfacets form during the LEO process there may be a significant variationin oxygen (or other dopant) level, giving rise to lateral variations inthe lattice constant and stresses that can cause cracking in the LEOcrystal itself or in a crystal grown on the latter, used as a seed.

Variations of the LEO method have been disclosed for other group IIImetal nitride growth techniques besides HVPE. In a first example, Jiang,et al. (U.S. No. 2014/0147650, now U.S. Pat. No. 9,589,792) disclosed aprocess for ammonothermal LEO growth of group-III metal nitrides,replacing the mask layer in typical vapor-phase LEO-type processes (SiO₂or SiN_(x)) by a combination of an adhesion layer, a diffusion-barrierlayer, and an inert layer. In a second example, Mori, et al. (U.S. No.2014/0328742, now U.S. Pat. No. 9,834,859) disclosed a process for LEOgrowth of group-III metal nitrides in a sodium-gallium flux. However, inthis method the coalescing crystallites typically have prominentsemipolar facets, leading to significant lateral variation in theimpurity content of coalesced crystals, and the thermal expansionmismatch between the coalesced nitride layer and a hetero-substrate,which includes a different material than the coalesced nitride layer,may cause uncontrolled cracking.

Several authors, for example, Linthicum et al. (Applied Physics Letters,75, 196, (1999)), Chen et al. (Applied Physics Letters 75, 2062 (1999)),and Wang, et al. (U.S. Pat. No. 6,500,257) have noted that threadingdislocations in growing GaN normally propagate predominantly in thegrowth direction and showed that the dislocation density can be reducedeven more than in the conventional LEO method by growing from thesidewalls of trenches in thin, highly-defective c-plane GaN layersrather than vertically through windows in a patterned mask. Thesemethods have been extended to nonpolar- and semipolar-oriented thin GaNfilms by other authors, for example, Chen et al. (Japanese Journal ofApplied Physics 42, L818 (2003)) and Imer et al. (U.S. Pat. No.7,361,576). However, to the best of the inventors' knowledge, sidewallLEO methods have not yet been extended to growth of bulk GaN, nor to thegrowth of N-sector GaN. In particular, we have found that differentmethods that those used in the thin film studies work best to formtrenches several hundred microns deep with pitches on the millimeterscale and produce some unexpected benefits.

FIGS. 1A-1E are schematic cross-sectional views of a seed crystal or asubstrate during various stages of a method for forming a patterned maskseed layer for ammonothermal sidewall lateral epitaxial overgrowth.Referring to FIG. 1A, a substrate 101 is provided with a photoresistlayer 103 disposed thereon. In certain embodiments, substrate 101consists of or includes a substrate material that is asingle-crystalline group-III metal nitride, gallium-containing nitride,or gallium nitride. The substrate 101 may be grown by HVPE,ammonothermally, or by a flux method. One or both large area surfaces ofsubstrate 101 may be polished and/or chemical-mechanically polished. Alarge-area surface 102 of substrate 101 may have a crystallographicorientation within 5 degrees, within 2 degrees, within 1 degree, orwithin 0.5 degree of (0001)+c-plane, (000-1) −c-plane, {10−10} m-plane,{11−2±2}, {60−6±1}, {50−5±1}, {40−4±1}, {30−3±1}, {50−5±2}, {70−7±3},{20−2±1}, {30−3±2}, {40−4±3}, {50−5±4}, {10−1±1}, {1 0−1±2}, {1 0−1±3},{2 1−3±1}, or {3 0−3±4}. It will be understood that plane {3 0−3±4}means the {3 0−3 4} plane and the {3 0−3 −4} plane. Large-area surface102 may have an (h k i l) semipolar orientation, where i=−(h+k) and land at least one of h and k are nonzero. Large-area surface 102 may havea maximum lateral dimension between about 5 millimeters and about 600millimeters and a minimum lateral dimension between about 1 millimeterand about 600 millimeters and substrate 101 may have a thickness betweenabout 10 micrometers and about 10 millimeters, or between about 100micrometers and about 2 millimeters.

Substrate 101 may have a surface threading dislocation density less thanabout 10⁷ cm⁻², less than about 10⁶ cm⁻², less than about 10⁵ cm⁻², lessthan about 10⁴ cm⁻², less than about 10³ cm⁻², or less than about 10²cm⁻². Substrate 101 may have a stacking-fault concentration below about10⁴ cm⁻¹, below about 10³ cm⁻¹, below about 10² cm⁻¹, below about 10cm⁻¹ or below about 1 cm⁻¹. Substrate 101 may have a symmetric x-rayrocking curve full width at half maximum (FWHM) less than about 500arcsec, less than about 300 arcsec, less than about 200 arcsec, lessthan about 100 arcsec, less than about 50 arcsec, less than about 35arcsec, less than about 25 arcsec, or less than about 15 arcsec.Substrate 101 may have a crystallographic radius of curvature greaterthan 0.1 meter, greater than 1 meter, greater than 10 meters, greaterthan 100 meters, or greater than 1000 meters, in at least one, at leasttwo, or in three independent or orthogonal directions.

Substrate 101 may comprise regions having a relatively highconcentration of threading dislocations separated by regions having arelatively low concentration of threading dislocations. Theconcentration of threading dislocations in the relatively highconcentration regions may be greater than about 10⁵ cm⁻², greater thanabout 10⁶ cm⁻², greater than about 10⁷ cm⁻², or greater than about 10⁸cm⁻². The concentration of threading dislocations in the relatively lowconcentration regions may be less than about 10⁶ cm⁻², less than about10⁵ cm⁻², or less than about 10⁴ cm⁻². Substrate 101 may compriseregions having a relatively high electrical conductivity separated byregions having a relatively low electrical conductivity. Substrate 101may have a thickness between about 10 microns and about 100 millimeters,or between about 0.1 millimeter and about 10 millimeters. Substrate 101may have a dimension, including a diameter, of at least about 5millimeters, at least about 10 millimeters, at least about 25millimeters, at least about 50 millimeters, at least about 75millimeters, at least about 100 millimeters, at least about 150millimeters, at least about 200 millimeters, at least about 300millimeters, at least about 400 millimeters, or at least about 600millimeters.

Large-area surface 102 (FIG. 1A) may have a crystallographic orientationwithin about 5 degrees of the (000-1) N-face, c-plane orientation, mayhave an x-ray diffraction w-scan rocking curvefull-width-at-half-maximum (FWHM) less than about 200 arcsec less thanabout 100 arcsec, less than about 50 arcsec, or less than about 30arcsec for the (002) and/or the (102) reflections and may have adislocation density less than about 10⁷ cm⁻², less than about 10⁶ cm⁻²,or less than about 10⁵ cm⁻². In some embodiments, the threadingdislocations in large-area surface 102 are approximately uniformlydistributed. In other embodiments, the threading dislocations inlarge-area surface 102 are arranged inhomogenously as a one-dimensionalarray of rows of relatively high- and relatively low-concentrationregions or as a two-dimensional array of high-dislocation-densityregions within a matrix of low-dislocation-density regions. Thecrystallographic orientation of large-area surface 102 may be constantto less than about 5 degrees, less than about 2 degrees, less than about1 degree, less than about 0.5 degree, less than about 0.2 degree, lessthan about 0.1 degree, or less than about 0.05 degree. In certainembodiments, large-area surface 102 is roughened to enhance adhesion ofa mask layer, for example, by wet-etching, to form a frosted morphology.

Referring again to FIG. 1A, a photoresist layer 103 may be deposited onthe large-area surface 102 by methods that are known in the art. Forexample, in a certain embodiment of a lift-off process, a liquidsolution of a negative photoresist is first applied to large-areasurface 102. Substrate 101 is then spun at a high speed (for example,between 1000 to 6000 revolutions per minute for 30 to 60 seconds),resulting in a uniform photoresist layer 103 on large-area surface 102.Photoresist layer 103 may then be baked (for example, between about 90and about 120 degrees Celsius) to remove excess photoresist solvent.After baking, the photoresist layer 103 may then be exposed to UV lightthrough a photomask (not shown) to form a patterned photoresist layer104 (FIG. 1B) having a pre-determined pattern of cross-linkedphotoresist, such as regions 104A, formed within the unexposed regions104B. The regions 104B of the patterned photoresist may form stripes ordots having characteristic width or diameter W and pitch L. Thepatterned photoresist layer 104 may then be developed to removenon-cross linked material found in regions 104B and leave regions 104A,such as illustrated in FIG. 1C.

Referring to FIG. 1D, one or more mask layers 111 may be deposited onlarge-area surface 102 and regions 104A of the patterned photoresistlayer 104. The one or more mask layers 111 may comprise an adhesionlayer 105 that is deposited on the large-area surface 102, adiffusion-barrier layer 107 deposited over the adhesion layer 105, andan inert layer 109 deposited over the diffusion-barrier layer 107. Theadhesion layer 105 may comprise one or more of Ti, TiN, TiN_(y), TiSi₂,Ta, TaN_(y), Al, Ge, Al_(x)Ge_(y), Cu, Si, Cr, V, Ni, W, TiW_(x),TiW_(x)N_(y), or the like and may have a thickness between about 1nanometer and about 1 micrometer. The diffusion-barrier layer 107 maycomprise one or more of TiN, TiN_(y), TiSi₂, W, TiW_(x), TiN_(y),WN_(y), TaN_(y), TiW_(x)N_(y), TiW_(x)Si_(z)N_(y), TiC, TiCN, Pd, Rh,Cr, or the like, and have a thickness between about 1 nanometer andabout 10 micrometers. The inert layer 109 may comprise one or more ofAu, Ag, Pt, Pd, Rh, Ru, Ir, Ni, Cr, V, Ti, or Ta and may have athickness between about 10 nanometers and about 100 micrometers. The oneor more mask layers 111 may be deposited by sputter deposition, thermalevaporation, electron-beam evaporation, or the like. After deposition ofthe patterned mask layer(s) 111, the portion of the mask layer(s) 111residing above the regions 104A of the patterned photoresist layer 104are not in direct contact with the substrate 101, as shown in FIG. 1D.The regions 104A and portions of the mask layer(s) 111 disposed thereonare then lifted off by methods that are known in the art to form theopenings 112 in the mask layer(s) 111, as shown in FIG. 1E. In certainembodiments, a relatively thin inert layer, for example, 10 to 500nanometers thick, is deposited prior to the lift-off process. Afterperforming the lift-off process, an additional, thicker inert layer, forexample, 5 to 100 micrometers thick, may be deposited over thealready-patterned inert layer by electroplating, electroless deposition,or the like.

Other methods besides the lift-off procedure described above may be usedto form the patterned mask layer 111, including shadow masking, positiveresist reactive ion etching, wet chemical etching, ion milling, andnanoimprint lithography, plus variations of the negative resist lift-offprocedure described above.

In certain embodiments, patterned mask layer(s) 111 are deposited onboth the front and back surfaces of substrate 101.

FIGS. 1F-1L are top views of arrangements of exposed regions 120 on thesubstrate 101 formed by one or more of the processed described above.The exposed regions 120 (or also referred to herein as growth centers),which are illustrated, for example, in FIGS. 1F-1L, may be defined bythe openings 112 formed in patterned mask layer(s) 111 shown in FIG. 1E.In certain embodiments, the exposed regions 120 are arranged in aone-dimensional (1D) array in the y-direction, such as a single columnof exposed regions 120 as shown in FIG. 11. In certain embodiments, theexposed regions 120 are arranged in a two-dimensional (2D) array in x-and y-directions, such as illustrated in FIGS. 1F-1H and 1J-1L. Theopenings 112, and thus exposed regions 120, may be round, square,rectangular, triangular, hexagonal, or the like, and may have an openingdimension or diameter W between about 1 micrometer and about 5millimeters, or between about 10 micrometers and about 500 micrometerssuch as illustrated in FIGS. 1F-1L. The exposed regions 120 may bearranged in a 2D hexagonal or square array with a pitch dimension Lbetween about 5 micrometers and about 20 millimeters, between about 200micrometers and about 15 millimeters, or between about 500 micrometersand about 10 millimeters, or between about 0.8 millimeter and about 5millimeters, such as illustrated in FIGS. 1F and 1G. The exposed regions120 may be arranged in a 2D array, in which the pitch dimension L₁ inthe y-direction and pitch dimension L₂ in the x-direction may bedifferent from one another, as illustrated in FIGS. 1H and 1J-1L. Theexposed regions 120 may be arranged in a rectangular, parallelogram,hexagonal, or trapezoidal array (not shown), in which the pitchdimensions L₁ in the y-direction and L₂ in the x-direction may bedifferent from one another, as illustrated in FIGS. 1H and 1J-1L). Thearray of exposed regions 120 may also be linear or irregular shaped. Theexposed regions 120 in patterned mask layer(s) 111 may be placed inregistry with the structure of substrate 101. For example, in certainembodiments, large-area surface 102 is hexagonal, e.g., a (0001) or(000-1) crystallographic orientation, and the openings in patterned masklayer(s) 111 comprise a 2D hexagonal array such that the separationsbetween nearest-neighbor openings are parallel to <11-20> or <10-10>directions in large-area surface 102. In certain embodiments, large-areasurface 102 of the substrate is nonpolar or semipolar and the exposedregions 120 comprise a 2D square or rectangular array such that theseparations between nearest-neighbor openings are parallel to theprojections of two of the c-axis, an m-axis, and an a-axis on large-areasurface 102 of substrate 101. In certain embodiments, the pattern ofexposed regions 120 is obliquely oriented with respect to the structureof substrate 101, for example, wherein the exposed regions 120 arerotated by between about 1 degree and about 44 degrees with respect to ahigh-symmetry axis of the substrate, such as a projection of the c-axis,an m-axis, or an a-axis on large-area surface 102 of substrate 101 thathas a hexagonal crystal structure, such as a Wurtzite crystal structure.In certain embodiments, the exposed regions 120 are substantially linearrather than substantially round. In certain embodiments, the exposedregions 120 are slits having a width W and period L that run across theentire length of substrate 101, as illustrated in FIG. 11. In certainembodiments, the exposed regions 120 are slits that have a width Wi inthe y-direction and a predetermined length W₂ in the x-direction that isless than the length of substrate 101 and may be arranged in a 2D lineararray with period L₁ in the y-direction and period L₂ in thex-direction, as illustrated in FIGS. 1J-1L. In some embodiments,adjacent rows of exposed regions 120 (e.g., slits) may be offset in thex-direction from one another rather than arranged directly adjacent, asshown in FIG. 1K. In certain embodiments, the adjacent rows of exposedregions 120 (e.g., slits) may be offset in the longitudinal y-directionfrom one another. In certain embodiments, the exposed regions 120include slits that extend in two or more different directions, forexample, the x-direction and the y-direction, as shown in FIG. 1L. Incertain embodiments, the exposed regions 120 (e.g., slits) may bearranged in a way that reflects the hexagonal symmetry of the substrate.In certain embodiments, the exposed regions 120 (e.g., slits) may extendto the edge of the substrate 101.

In an alternative embodiment, as shown in FIG. 1M, large-area surface102 of substrate 101 is covered with a blanket mask 116, comprising one,two, or more of adhesion layer 105, diffusion-barrier layer 107, andinert layer 109, followed by a positive photoresist layer 113. Thephotoresist layer is exposed to UV light through a photomask (notshown), forming solubilizable, exposed regions 106B and unexposedregions 106A, as shown in FIG. 1N (essentially the negative of thepattern shown in FIG. 1B). Exposed regions 106B are then removed bydeveloping. As shown in FIG. 1O, openings 112 in the blanket mask 116(comprising adhesion layer 105, diffusion-barrier layer 107, and inertlayer 109) may then be formed by wet or dry etching through the openingsin patterned photoresist layer 113A, to form the patterned mask layer111. After forming the openings 112 the photoresist layer 113 isremoved, as shown in FIG. 1P, producing a structure that is similar oridentical to that shown in FIG. 1E.

Trenches 115 are then formed in exposed regions 120 of the substrate 101through the openings 112 (or “windows”) formed in patterned mask layer111, as shown in FIG. 1Q. In certain embodiments, the depth of thetrenches 115 is between 50 micrometers and about 1 millimeter or betweenabout 100 micrometers and about 300 micrometers. In certain embodimentsthe trenches 115 penetrate the entire thickness of substrate 101,forming patterned holes or slits that extend from the rear side 118 ofthe substrate 101 and through the openings 112 of the patterned masklayer 111. The width of an individual trench may be between about 10micrometers and about 500 micrometers, or between about 20 micrometersand about 200 micrometers. Individual trenches 115 may be linear orcurved and may have a length in the X-direction and/or Y-directionbetween about 100 micrometers and about 50 millimeters, or between about200 micrometers and about 10 millimeters, or between about 500micrometers and about 5 millimeters. In a specific embodiment,large-area surface 102 of substrate 101 has a (000-1), N-faceorientation and trench 115 is formed by wet etching. In a specificembodiment, an etchant composition or solution comprises a solution of85% phosphoric acid (H₃PO₄) and sulfuric (H₂SO₄) acids with aH₂SO₄/H₃PO₄ ratio between 0 and about 1:1. In certain embodiments, aphosphoric acid solution is conditioned to form polyphosphoric acid,increasing its boiling point. For example, reagent-grade (85%) H₃PO₄ maybe conditioned by stirring and heating in a beaker at a temperaturebetween about 200 degrees Celsius and about 450 degrees Celsius forbetween about 5 minutes and about five hours. In a specific embodiment,trench 115 is formed by heating masked substrate 101 in one of theaforementioned etch solutions at a temperature between about 200 degreesCelsius and about 350 degrees Celsius for a time between about 15minutes and about 6 hours. In another embodiment, trench 115 is formedby electrochemical wet etching.

FIGS. 1R-1T show an alternative approach to forming an array ofpatterned, masked trenches in substrate 101. A blanket mask 116(comprising adhesion layer 105, diffusion-barrier layer 107, and inertlayer 109) may be deposited on large-area surface 102 of substrate 101as shown in FIG. 1R. Nascent trenches 114 may be formed by laserablation, as shown in FIG. 1S, to form a patterned mask layer 111. Thelaser ablation process is also known as or referred to as lasermachining or laser beam machining processes. Laser ablation may beperformed by a watt-level laser, such as a neodymium-dopedyttrium-aluminum-garnet (Nd:YAG) laser, a CO₂ laser, an excimer laser, aTi:sapphire laser, or the like. The laser may emit pulses with a pulselength in the nanosecond, picosecond, or femtosecond range. In certainembodiments the frequency of the output light of the laser may bedoubled, tripled, or quadrupled using an appropriate nonlinear optic.The beam width, power, and scan rate of the laser over the surface ofsubstrate 101 with patterned mask layer 111 may be varied to adjust thewidth, depth, and aspect ratio of nascent trenches 114. The laser may bescanned repetitively over a single trench or repetitively over the wholearray of trenches.

The surfaces and sidewalls of the nascent trenches 114 may containdamage left over from the laser ablation process. In certainembodiments, substrate 101, containing nascent trenches 114, is furtherprocessed by wet etching, dry etching, or photoelectrochemical etchingin order to remove residual damage in nascent trenches 114 as shown inFIG. 1T. In a specific embodiment, large-area surface 102 of substrate101 has a (000-1), N-face orientation and a trench 115 is formed fromnascent trench 114 by wet etching. In a specific embodiment, an etchantcomposition or solution comprises a solution of 85% phosphoric acid(H₃PO₄) and sulfuric (H₂SO₄) acids with a H₂SO₄/H₃PO₄ ratio between 0and about 1:1. In certain embodiments, a phosphoric acid solution isconditioned to form polyphosphoric acid, increasing its boiling point.For example, reagent-grade (85%) H₃PO₄ may be conditioned by stirringand heating in a beaker at a temperature between about 200 degreesCelsius and about 450 degrees Celsius for between about 5 minutes andabout five hours. In a specific embodiment, trench 115 is formed byheating substrate 101 in one of the aforementioned etch solutions at atemperature between about 200 degrees Celsius and about 350 degreesCelsius for a time between about 15 minutes and about 6 hours.

The substrate 101 with the masked, patterned trenches 115 is then usedas a substrate for bulk crystal growth, for example, comprisingammonothermal growth, HVPE growth, or flux growth. In the discussionbelow the grown GaN layer will be referred to as an ammonothermal layer,even though other bulk growth methods, such as HVPE or flux growth, maybe used instead. In certain embodiments, comprising ammonothermal bulkgrowth, patterned substrate 101 may then be suspended on a seed rack andplaced in a sealable container, such as a capsule, an autoclave, or aliner within an autoclave. In certain embodiments, one or more pairs ofpatterned substrates are suspended back to back, with the patternedlarge area surfaces facing outward. A group III metal source, such aspolycrystalline group III metal nitride, at least one mineralizercomposition, and ammonia (or other nitrogen containing solvent) are thenadded to the sealable container and the sealable container is sealed.The mineralizer composition may comprise an alkali metal such as Li, Na,K, Rb, or Cs, an alkaline earth metal, such as Mg, Ca, Sr, or Ba, or analkali or alkaline earth hydride, amide, imide, amido-imide, nitride, orazide. The mineralizer may comprise an ammonium halide, such as NH₄F,NH₄Cl, NH₄Br, or NH₄I, a gallium halide, such as GaF₃, GaCl₃, GaBr₃,GaI₃, or any compound that may be formed by reaction of one or more ofF, Cl, Br, I, HF, HCl, HBr, HI, Ga, GaN, and NH₃. The mineralizer maycomprise other alkali, alkaline earth, or ammonium salts, other halides,urea, sulfur or a sulfide salt, or phosphorus or a phosphorus-containingsalt. The sealable container (e.g., capsule) may then be placed in ahigh pressure apparatus, such as an internally heated high pressureapparatus or an autoclave, and the high pressure apparatus sealed. Thesealable container, containing patterned substrate 101, is then heatedto a temperature above about 400 degrees Celsius and pressurized aboveabout 50 megapascal to perform ammonothermal crystal growth.

As a point of reference, FIGS. 2A-2C illustrate bulk crystal growth by aconventional LEO process with no trenches below mask openings. During abulk crystal growth process, group III metal nitride layer 213 growsthrough the openings 112 of patterned mask layer 111, grows outwardthrough the openings, as shown in FIG. 2B, grows laterally overpatterned mask layer 111, and coalesces, as shown in FIG. 2C. Aftercoalescence, group III metal nitride layer 213 comprises window regions215, which have grown vertically with respect to the openings inpatterned mask layer 111, wing regions 217, which have grown laterallyover patterned mask layer 111, and coalescence fronts 219, which form atthe boundaries between wings growing from adjacent openings in patternedmask layer 111. Threading dislocations 214 may be present in windowregions 215, originating from threading dislocations that were presentat the surface of the substrate 101.

FIGS. 3A-3C illustrate a bulk group III nitride sidewall LEO process.FIG. 3A illustrates a substrate that includes a patterned, masked trench115, formed by one of the processes described herein. In a sidewall LEOprocess, a group III metal nitride material 221 grows on the sides andbottoms of the patterned, masked trenches 115 as shown in FIG. 3B. Asgroup III metal nitride material 221 on the sidewalls of trenches 115grow inward, it becomes progressively more difficult for group IIInitride nutrient material to reach the bottom of the trenches, whetherthe nutrient material comprises an ammonothermal complex of a group IIImetal (in the case of ammonothermal growth), a group III metal halide(in the case of HVPE), or a group III metal alloy or inorganic complex(in the case of flux growth). Eventually group III metal nitridematerial 221 pinches off the lower regions of the trenches, formingvoids 225 as shown in FIG. 3C. It has been found that the concentrationof threading dislocations in group III metal nitride material 221, whichhas grown laterally, is lower than that in substrate 101. Many threadingdislocations 223, originating from substrate 101, terminate on thesurfaces of voids 225. Concomitantly, the group III metal nitride layer213 grows upward through openings 112 (or windows) in patterned masklayer 111. However, since laterally-grown group III metal nitridematerial 221 has a lower concentration of threading dislocations thansubstrate 101 and many dislocations from substrate 101 have terminatedat surfaces of voids 225, the dislocation density in the verticallygrown group III metal nitride layer 213 is considerably reduced,relative to a conventional LEO process, as described above inconjunction with FIG. 2A-2C.

FIGS. 3D-3F illustrate the continuation of the sidewall LEO growthprocess. As in the conventional LEO process (FIGS. 2A-2C), group IIImetal nitride layer 213 grows within the openings 112 of patterned masklayer 111, grows outward through the openings as shown in FIG. 3D, growslaterally over patterned mask layer 111, and coalesces, as shown in FIG.3E. After coalescence, group III metal nitride layer 213 compriseswindow regions 215, which have grown vertically with respect to theopenings in patterned mask layer 111, wing regions 217, which have grownlaterally over patterned mask layer 111, and coalescence fronts 219,which form at the boundaries between wings growing from adjacentopenings in patterned mask layer 111, as shown in FIG. 3F. Sincelaterally-grown group III metal nitride material 221 has a lowerconcentration of threading dislocations than substrate 101 and manythreading dislocations from substrate 101 have terminated in voids 225,the concentration of threading dislocations in window regions 215 issignificantly lower than in the case of conventional LEO.

Ammonothermal group III metal nitride layer 213 may have a thicknessbetween about 10 micrometers and about 100 millimeters, or between about100 micrometers and about 20 millimeters.

In certain embodiments, ammonothermal group III metal nitride layer 213is subjected to one or more processes, such as at least one of sawing,lapping, grinding, polishing, chemical-mechanical polishing, or etching.

In certain embodiments, the concentration of extended defects, such asthreading dislocations and stacking faults, in the ammonothermal groupIII metal nitride layer 213 may be quantified by defect selectiveetching. Defect-selective etching may be performed, for example, using asolution comprising one or more of H₃PO₄, H₃PO₄ that has beenconditioned by prolonged heat treatment to form polyphosphoric acid, andH₂SO₄, or a molten flux comprising one or more of NaOH and KOH.Defect-selective etching may be performed at a temperature between about100 degrees Celsius and about 500 degrees Celsius for a time betweenabout 5 minutes and about 5 hours, wherein the processing temperatureand time are selected so as to cause formation of etch pits withdiameters between about 1 micrometer and about 25 micrometers, thenremoving the ammonothermal group III metal nitride layer, crystal, orwafer from the etchant solution.

The concentration of threading dislocations in the surface of the windowregions 215 may be less than that in the underlying substrate 101 by afactor between about 10 and about 10⁴. The concentration of threadingdislocations in the surface of the window regions 215 may be less thanabout 10⁸ cm⁻², less than about 10⁷ cm⁻², less than about 10⁶ cm⁻², lessthan about 10⁵ cm⁻², or less than about 10⁴ cm⁻². The concentration ofthreading dislocations in the surface of wing regions 217 may be lower,by about one to about three orders of magnitude, than the concentrationof threading dislocations in the surface of the window regions 215, andmay be below about 10⁵ cm⁻², below about 10⁴ cm⁻², below about 10³ cm⁻²,below about 10² cm⁻², or below about 10 cm⁻². Some stacking faults, forexample, at a concentration between about 1 cm⁻¹ and about 10⁴ cm⁻¹, maybe present at the surface of the window regions 215. The concentrationof stacking faults in the surface of wing regions 217 may be lower, byabout one to about three orders of magnitude, than the concentration ofstacking faults in the surface of the window regions 215, and may bebelow about 10² cm⁻¹, below about 10 cm⁻¹, below about 1 cm⁻¹, or belowabout 0.1 cm⁻¹, or may be undetectable. Threading dislocations, forexample, edge dislocations, may be present at coalescence fronts 219,for example, with a line density that is less than about 1×10⁵ cm⁻¹,less than about 3×10⁴ cm⁻¹, less than about 1×10⁴ cm⁻¹, less than about3×10³ cm⁻¹, less than about 1×10³ cm⁻¹, less than about 3×10² cm⁻¹, orless than 1×10² cm⁻¹. The density of dislocations along the coalescencefronts may be greater than 5 cm⁻¹, greater than 10 cm⁻¹, greater than 20cm⁻¹, greater than 50 cm⁻¹, greater than 100 cm⁻¹, greater than 200cm⁻¹, or greater than 500 cm⁻¹.

In certain embodiments, the process of masking and bulk group IIInitride crystal growth is repeated one, two, three, or more times. Insome embodiments, these operations are performed while the first bulkgroup III metal nitride layer remains coupled to substrate 101. In otherembodiments, substrate 101 is removed prior to a subsequent masking andbulk crystal growth operation, for example, by sawing, lapping,grinding, and/or etching.

FIGS. 4A and 4B are simplified diagrams illustrating a method of forminga free-standing group III metal nitride boule and free-standing groupIII metal nitride wafers. In certain embodiments, substrate 101 isremoved from ammonothermal group III metal nitride layer 213 (FIG. 3F),or the last such layer deposited, to form free-standing ammonothermalgroup III metal nitride boule 413. Removal of substrate 101 may beaccomplished by one or more of sawing, grinding, lapping, polishing,laser lift-off, self-separation, and etching to form a processedfree-standing laterally-grown group III metal nitride boule 413. Theprocessed free-standing laterally-grown group III metal nitride boule413 may include a similar or essentially identical composition as theammonothermal group III metal nitride layer and etching may be performedunder conditions where the etch rate of the back side of substrate 101is much faster than the etch rate of the front surface of theammonothermal group III metal nitride layer. In certain embodiments aportion of ammonothermal group III metal nitride layer 213, or the lastsuch layer deposited, may be protected from attack by the etchant bydeposition of a mask layer, wrapping the portion of the layer withTeflon, clamping the portion of the layer against Teflon, painting withTeflon paint, or the like. In a specific embodiment, substrate 101comprises single crystal gallium nitride, large-area surface 102 ofsubstrate 101 has a crystallographic orientation within about 5 degreesof a (0001) crystallographic orientation, and substrate 101 ispreferentially etched by heating in a solution comprising one or more ofH₃PO₄, H₃PO₄ that has been conditioned by prolonged heat treatment toform polyphosphoric acid, and H₂SO₄ at a temperature between about 150degrees Celsius and about 500 degrees Celsius for a time between about30 minutes and about 5 hours, or by heating in a molten flux comprisingone or more of NaOH and KOH. Surprisingly, patterned mask layer(s) 111may facilitate preferential removal of substrate 101 by acting as anetch stop. The processed free-standing ammonothermal group III metalnitride boule 413 may include one or more window regions 415 that wereformed above exposed regions 120, such as openings 112 in patterned masklayer(s) 111, on a substrate 101. The processed free-standinglaterally-grown group III metal nitride boule 413 may also include oneor more wing regions 417 that were formed above non-open regions inpatterned mask layer(s) 111, and a pattern oflocally-approximately-linear arrays 419 of threading dislocations, asshown in FIG. 4A. One or more of front surface 421 and back surface 423of free-standing ammonothermal group III metal nitride boule 413 may belapped, polished, etched, and chemical-mechanically polished. Assimilarly discussed above, the coalescence fronts 419 may include acoalescence front region that includes a “sharp boundary” that has awidth less than about 25 micrometers or less than about 10 micrometersthat is disposed between the adjacent wing regions 417, or an “extendedboundary” that has a width between about 25 micrometers and about 1000micrometers or between about 30 micrometers and about 250 micrometersthat is disposed between the adjacent wing regions 417, depending on thegrowth conditions.

In certain embodiments, the edge of free-standing ammonothermal groupIII metal nitride boule 413 is ground to form a cylindrically-shapedammonothermal group III metal nitride boule. In certain embodiments, oneor more flats is ground into the side of free-standing ammonothermalgroup III metal nitride boule 413. In certain embodiments, free-standingammonothermal group III metal nitride boule 413 is sliced into one ormore free-standing ammonothermal group III metal nitride wafers 431, asshown in FIG. 4B. The slicing may be performed by multi-wire sawing,multi-wire slurry sawing, slicing, inner-diameter sawing, outer-diametersawing, cleaving, ion implantation followed by exfoliation, lasercutting, or the like. One or more large-area surface of free-standingammonothermal group III metal nitride wafers 431 may be lapped,polished, etched, electrochemically polished, photoelectrochemicallypolished, reactive-ion-etched, and/or chemical-mechanically polishedaccording to methods that are known in the art. In certain embodiments,a chamfer, bevel, or rounded edge is ground into the edges offree-standing ammonothermal group III metal nitride wafers 431. Thefree-standing ammonothermal group III metal nitride wafers may have adiameter of at least about 5 millimeters, at least about 10 millimeters,at least about 25 millimeters, at least about 50 millimeters, at leastabout 75 millimeters, at least about 100 millimeters, at least about 150millimeters, at least about 200 millimeters, at least about 300millimeters, at least about 400 millimeters, or at least about 600millimeters and may have a thickness between about 50 micrometers andabout 10 millimeters or between about 150 micrometers and about 1millimeter. One or more large-area surface of free-standingammonothermal group III metal nitride wafers 431 may be used as asubstrate for group III metal nitride growth by chemical vapordeposition, metalorganic chemical vapor deposition, hydride vapor phaseepitaxy, molecular beam epitaxy, flux growth, solution growth,ammonothermal growth, among others, or the like.

FIGS. 5A-5E are simplified diagrams illustrating threading dislocationpatterns formed in a free-standing group III metal nitride boule 413 orwafer 431. The large-area surfaces of the free-standing ammonothermalgroup III metal nitride boule 413 or wafers 431 may be characterized bya pattern of locally-approximately-linear arrays 419 of threadingdislocations that propagated from coalescence fronts 219 formed duringthe epitaxial lateral overgrowth process, as discussed above inconjunction with FIGS. 3A-3F. The pattern oflocally-approximately-linear arrays of threading dislocations may be 2Dhexagonal, square, rectangular, trapezoidal, triangular, 1D linear, oran irregular pattern that is formed at least partially due to thepattern of the exposed regions 120 (FIGS. 1F-1L) used during the processto form free-standing laterally-grown group III metal nitride boule 413.One or more window regions 415 are formed above the exposed regions 120(FIGS. 1F-1L), and one or more wing regions 417 are formed on portionsthat are not above the exposed regions 120, that is, were formed bylateral growth. As discussed above, the formed coalescence fronts 219 or419 may include coalescence front regions that have a lateral width(i.e., measured parallel to the surface of the page containing FIGS.5A-5E) that can vary depending on the growth conditions.

More complex patterns are also possible and may be advantageous, forexample, in being more resistant to cracking or cleaving. The pattern502 may be elongated in one direction compared to another orthogonaldirection, for example, due to the free-standing laterally-grown groupIII metal nitride boule 413 being sliced at an inclined angle relativeto the large-area surface of a free-standing ammonothermal group IIImetal nitride boule 413. The pattern 502 of locally-approximately-lineararrays of threading dislocations may be characterized by a linear arrayof threading dislocations (FIG. 5D) that have a pitch dimension Lbetween about 5 micrometers and about 20 millimeters or between about200 micrometers and about 5 millimeters. The pattern 502 oflocally-approximately-linear arrays of threading dislocations may becharacterized by a pitch dimension L (FIGS. 5A, 5B), or by pitchdimensions L₁ and L₂ in two orthogonal directions (FIGS. 5C and 5E),between about 5 micrometers and about 20 millimeters or between about200 micrometers and about 5 millimeters, or between about 500micrometers and about 2 millimeters. In certain embodiments, the pattern502 of locally-approximately-linear arrays of threading dislocations isapproximately aligned with the underlying crystal structure of the groupIII metal nitride, for example, with the locally-approximately-lineararrays lying within about 5 degrees, within about 2 degrees, or withinabout 1 degree of one or more of <1 0−1 0>, <1 1−2 0>, or [0 0 0±1] ortheir projections in the plane of the surface of the free-standingammonothermal group III metal nitride boule 413 or wafer 431. The linearconcentration of threading dislocations in the pattern may be less thanabout 1×10⁵ cm⁻¹, less than about 3×10⁴ cm⁻¹, less than about 1×10⁴cm⁻¹, less than about 3×10³ cm⁻¹, less than about 1×10³ cm⁻¹, less thanabout 3×10² cm⁻¹, or less than about 1×10² cm⁻¹. The linearconcentration of threading dislocations in the pattern 502 may begreater than 5 cm⁻¹, greater than 10 cm⁻¹, greater than 20 cm⁻¹, greaterthan 50 cm⁻¹, greater than 100 cm⁻¹, greater than 200 cm⁻¹, or greaterthan 500 cm⁻¹.

Referring again to FIGS. 5A-5E, the large-area surfaces of thefree-standing ammonothermal group III metal nitride boule or wafer mayfurther be characterized by an array of wing regions 417 and by an arrayof window regions 415. Each wing region 417 may be positioned betweenadjacent locally-approximately-linear arrays 419 of threadingdislocations. Each window region 415 may be positioned within a singlewing region 417 or may be positioned between two adjacent wing regions417 and may have a minimum dimension between 10 micrometers and 500micrometers and be characterized by concentration of threadingdislocations between 10³ cm⁻² and 10⁸ cm⁻², resulting from residualthreading dislocations that propagated vertically from window regionsduring the bulk crystal growth process, and by a concentration ofstacking faults below 10³ cm⁻¹. In some embodiments the boundary betweenthe window regions and the wing regions may be decorated withdislocations, for example, with a line density between about 5 cm⁻¹ and10⁵ cm⁻¹.

The arrays may be elongated in one direction compared to anotherorthogonal direction, for example, due to the boule being sliced at aninclined angle relative to the large-area surface of a free-standingammonothermal group III metal nitride boule. The pattern oflocally-approximately-linear arrays 419 of threading dislocations may becharacterized by a pitch dimension L, or by pitch dimensions L₁ and L₂in two orthogonal directions, between about 5 micrometers and about 20millimeters or between about 200 micrometers and about 5 millimeters. Incertain embodiments, the pattern of locally-approximately-linear arrays419 of threading dislocations is approximately aligned with theunderlying crystal structure of the group III metal nitride, forexample, with the locally-approximately-linear arrays lying within about5 degrees, within about 2 degrees, or within about 1 degree of one ormore of <1 0-1 0>, <1 1-2 0>, or [0 0 0±1] or their projections in theplane of the surface of the free-standing ammonothermal group IIInitride boule or wafer. The linear concentration of threadingdislocations in the pattern may be less than about 1×10⁵ cm⁻¹, less thanabout 3×10⁴ cm⁻¹, less than about 1×10⁴ cm⁻¹, less than about 3×10³cm⁻¹, less than about 1×10³ cm⁻¹, less than about 3×10² cm⁻¹, or lessthan about 1×10² cm⁻¹. The linear concentration of threadingdislocations in the pattern may be greater than 5 cm⁻¹, greater than 10cm⁻¹, greater than 20 cm⁻¹, greater than 50 cm⁻¹, greater than 100 cm⁻¹,greater than 200 cm⁻¹, or greater than 500 cm⁻¹.

The concentration of threading dislocations in the wing regions 417between the locally-approximately-linear arrays of threadingdislocations may be below about 10⁵ cm⁻², below about 10⁴ cm⁻², belowabout 10³ cm⁻², below about 10² cm⁻¹, or below about 10 cm⁻². Theconcentration of threading dislocations in the surface of the windowregions 415 may be less than about 10⁸ cm⁻², less than about 10⁷ cm⁻²,less than about 10⁶ cm⁻², less than about 10⁵ cm⁻², or less than about10⁴ cm⁻². The concentration of threading dislocations in the surface ofthe window regions may be higher than the concentration of threadingdislocations in the surface of the wing regions by at least a factor oftwo, by at least a factor of three, by at least a factor of ten, by atleast a factor of 30, or by at least a factor of 100. The concentrationof threading dislocations in the surface of the window regions may behigher than concentration of threading dislocations in the surface ofthe wing regions by less than a factor of 10⁴, by less than a factor of3000, by less than a factor of 1000, by less than a factor of 300, byless than a factor of 100, or by less than a factor of 30. In someembodiments the boundary between the window regions 415 and the wingregions 417 may be decorated with dislocations, for example, with a linedensity between about 5 cm⁻¹ and 10⁵ cm⁻¹. The concentration ofthreading dislocations, averaged over a large area surface of thefree-standing ammonothermal group III nitride boule or wafer, may bebelow about 10⁷ cm⁻², below about 10⁶ cm⁻², below about 10⁵ cm⁻², belowabout 10⁴ cm⁻², below about 10³ cm⁻², or below about 10² cm⁻². Theconcentration of stacking faults, averaged over a large area surface ofthe free-standing ammonothermal group III nitride boule or wafer, may bebelow about 10³ cm⁻¹, below about 10² cm⁻¹, below about 10 cm⁻¹, belowabout 1 cm⁻¹, or below about 0.1 cm⁻¹, or may be undetectable. In someembodiments, for example, after repeated re-growth on a seed crystalwith a patterned array of dislocations and/or growth to a thicknessgreater than 2 millimeters, greater than 3 millimeters, greater than 5millimeters, or greater than 10 millimeters, the positions of thethreading dislocations may be displaced laterally to some extent withrespect to the pattern on the seed crystal. In such a case the regionswith a higher concentration of threading dislocations may be somewhatmore diffuse than the relatively sharp lines illustrated schematicallyin FIGS. 5A-5E. However, the concentration of threading dislocations asa function of lateral position along a line on the surface will varyperiodically, with a period between about 5 micrometers and about 20millimeters or between about 200 micrometers and about 5 millimeters.The concentration of threading dislocations within theperiodically-varying region may vary by at least a factor of two, atleast a factor of 5, at least a factor of 10, at least a factor of 30,at least a factor of 100, at least a factor of 300, or at least a factorof 1000.

The free-standing ammonothermal group III metal nitride boule or wafermay have a large-area crystallographic orientation within 5 degrees,within 2 degrees, within 1 degree, within 0.5 degree, within 0.2 degree,within 0.1 degree, within 0.05 degree, within 0.02 degree, or within0.01 degree of (0001)+c-plane, (000-1)−c-plane, {10-10} m-plane, {1 1−20} a-plane, {11−2±2}, {60−6±1}, {50−5±1}, {40−4±1}, {30−3±1}, {50−5±2},{70−7±3}, {20−2±1}, {30−3±2}, {40−4±3}, {50−5±4}, {10−1±1}, {1 0−1±2},{1 0−1±3}, {2 1 −3±1}, or {3 0-3±4}. The free-standing ammonothermalgroup III metal nitride boule or wafer may have an (h k i l) semipolarlarge-area surface orientation, where i=−(h+k) and l and at least one ofh and k are nonzero.

In certain embodiments, a large-area surface of a free-standingammonothermal group III metal nitride crystal or wafer has acrystallographic orientation that is miscut from {10-10} m-plane bybetween about −60 degrees and about +60 degrees toward [0001]+c-direction and by up to about 10 degrees toward an orthogonal <1-210>a-direction. In certain embodiments, a large-area surface of thefree-standing ammonothermal group III metal nitride crystal or wafer hasa crystallographic orientation that is miscut from {10-10} m-plane bybetween about −30 degrees and about +30 degrees toward [0001]+c-direction and by up to about 5 degrees toward an orthogonal <1-210>a-direction. In certain embodiments, a large-area surface of thefree-standing ammonothermal group III metal nitride crystal or wafer hasa crystallographic orientation that is miscut from {10-10} m-plane bybetween about −5 degrees and about +5 degrees toward [0001] +c-directionand by up to about 1 degree toward an orthogonal <1-210> a-direction.The free-standing ammonothermal group III metal nitride crystal or wafermay have a stacking fault concentration below 10² cm⁻¹, below 10 cm⁻¹,or below 1 cm⁻¹, and a very low dislocation density, below about 10⁵cm⁻², below about 10⁴ cm⁻², below about 10³ cm⁻², below about 10² cm⁻²,or below about 10 cm⁻² on one or both of the two large area surfaces.

The free-standing ammonothermal group III metal nitride boule or wafermay have a symmetric x-ray rocking curve full width at half maximum(FWHM) less than about 200 arcsec, less than about 100 arcsec, less thanabout 50 arcsec, less than about 35 arcsec, less than about 25 arcsec,or less than about 15 arcsec. The free-standing ammonothermal group IIImetal nitride boule or wafer may have a crystallographic radius ofcurvature greater than 0.1 meter, greater than 1 meter, greater than 10meters, greater than 100 meters, or greater than 1000 meters, in atleast one, at least two, or in three independent or orthogonaldirections.

In certain embodiments, at least one surface of the free-standingammonothermal group III metal nitride boule or wafer has atomic impurityconcentrations of at least one of oxygen (O), and hydrogen (H) aboveabout 1×10¹⁶ cm⁻³, above about 1×10¹⁷ cm⁻³, or above about 1×10¹⁸ cm⁻³.In certain embodiments, a ratio of the atomic impurity concentration ofH to the atomic impurity concentration of 0 is between about 0.3 andabout 1000, between about 0.4 and about 10, or between about 10 andabout 100. In certain embodiments, at least one surface of thefree-standing ammonothermal group III metal nitride boule or wafer hasimpurity concentrations of at least one of lithium (Li), sodium (Na),potassium (K), fluorine (F), chlorine (CI), bromine (Br), or iodine (I)above about 1×10¹⁵ cm⁻³, above about 1×10¹⁶ cm⁻³, or above about 1×10¹⁷cm⁻³, above about 1×10¹⁸ cm⁻³. In certain embodiments, the top andbottom surfaces of the free-standing ammonothermal group III metalnitride boule or wafer may have impurity concentrations of O, H, carbon(C), Na, and K between about 1×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³, between about1×10¹⁶ cm⁻³ and 2×10¹⁹ cm⁻³, below 1×10¹⁷ cm⁻³, below 1×10¹⁶ cm⁻³, andbelow 1×10¹⁶ cm⁻³, respectively, as quantified by calibrated secondaryion mass spectrometry (SIMS). In another embodiment, the top and bottomsurfaces of the free-standing ammonothermal group III metal nitrideboule or wafer may have impurity concentrations of O, H, C, and at leastone of Na and K between about 1×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³, between about1×10¹⁶ cm⁻³ and 2×10¹⁹ cm⁻³, below 1×10¹⁷ cm⁻³, and between about 3×10¹⁵cm⁻³ and 1×10¹⁸ cm⁻³, respectively, as quantified by calibratedsecondary ion mass spectrometry (SIMS). In still another embodiment, thetop and bottom surfaces of the free-standing ammonothermal group IIImetal nitride boule or wafer may have impurity concentrations of O, H,C, and at least one of F and CI between about 1×10¹⁶ cm⁻³ and 1×10¹⁹cm⁻³, between about 1×10¹⁶ cm⁻³ and 2×10¹⁹ cm⁻³, below 1×10¹⁷ cm⁻³, andbetween about 1×10¹⁵ cm⁻³ and 1×10¹⁹ cm⁻³, respectively, as quantifiedby calibrated secondary ion mass spectrometry (SIMS). In someembodiments, the top and bottom surfaces of the free-standingammonothermal group III metal nitride boule or wafer may have impurityconcentrations of H between about 5×10¹⁷ cm⁻³ and 1×10¹⁹ cm⁻³, asquantified by calibrated secondary ion mass spectrometry (SIMS). Incertain embodiments, at least one surface of the free-standingammonothermal group III metal nitride boule or wafer has an impurityconcentration of copper (Cu), manganese (Mn), and iron (Fe) betweenabout 1 x10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³. In a specific embodiment, thefree-standing ammonothermal group III metal nitride boule or wafer hasan infrared absorption peak at about 3175 cm⁻¹, with an absorbance perunit thickness of greater than about 0.01 cm⁻¹.

The free-standing ammonothermal group III metal nitride crystal or wafermay be characterized by a wurtzite structure substantially free from anycubic entities or other crystal structures, the other structures beingless than about 0.1% in volume in reference to the substantiallywurtzite structure.

Surprisingly, given the lattice mismatch between HVPE GaN andammonothermal GaN, results of use of the herein-disclosed techniquesshow that ammonothermal lateral epitaxial overgrowth is capable ofproducing thick, large-area GaN layers that are free of cracks. Incertain embodiments, the free-standing ammonothermal group III metalnitride crystal or wafer has a diameter larger than about 25millimeters, larger than about 50 millimeters, larger than about 75millimeters, larger than about 100 millimeters, larger than about 150millimeters, larger than about 200 millimeters, larger than about 300millimeters, or larger than about 600 millimeters, and a thicknessgreater than about 0.1 millimeter, greater than about 0.2 millimeter,greater than about 0.3 millimeter, greater than about 0.5 millimeter,greater than about 1 millimeter, greater than about 2 millimeters,greater than about 3 millimeters, greater than about 5 millimeters,greater than about 10 millimeters, or greater than about 20 millimeters,and is substantially free of cracks. By contrast, we find thatammonothermal growth on large-area, un-patterned HVPE GaN seed crystalsleads to cracking if the layers are thicker than a few hundred microns,even if a patterning process had been used to form the HVPE GaN seedcrystal.

A free-standing ammonothermal group III metal nitride wafer may becharacterized by a total thickness variation (TTV) of less than about 25micrometers, less than about 10 micrometers, less than about 5micrometers, less than about 2 micrometers, or less than about 1micrometer, and by a macroscopic bow that is less than about 200micrometers, less than about 100 micrometers, less than about 50micrometers, less than about 25 micrometers, or less than about 10micrometers. A large-area surface of the free-standing ammonothermalgroup III metal nitride wafer may have a concentration of macro defects,with a diameter or characteristic dimension greater than about 100micrometers, of less than about 2 cm⁻², less than about 1 cm⁻², lessthan about 0.5 cm⁻², less than about 0.25 cm⁻², or less than about 0.1cm⁻². The variation in miscut angle across a large-area surface of thefree-standing ammonothermal group III metal nitride crystal or wafer maybe less than about 5 degrees, less than about 2 degrees, less than about1 degree, less than about 0.5 degree, less than about 0.2 degree, lessthan about 0.1 degree, less than about 0.05 degree, or less than about0.025 degree in each of two orthogonal crystallographic directions. Theroot-mean-square surface roughness of a large-area surface of thefree-standing ammonothermal group III metal nitride wafer, as measuredover an area of at least 10 μm×10 μm, may be less than about 0.5nanometer, less than about 0.2 nanometer, less than about 0.15nanometer, less than about 0.1 nanometer, or less than about 0.10nanometer. The free-standing ammonothermal group III metal nitride wafermay be characterized by n-type electrical conductivity, with a carrierconcentration between about 1×10¹⁷ cm⁻³ and about 3×10¹⁹ cm⁻³ and acarrier mobility greater than about 100 cm²/V-s. In alternativeembodiments, the free-standing ammonothermal group III metal nitridewafer is characterized by p-type electrical conductivity, with a carrierconcentration between about 1×10¹⁵ cm⁻³ and about 1×10¹⁹ cm⁻³. In stillother embodiments, the free-standing ammonothermal group III metalnitride wafer is characterized by semi-insulating electrical behavior,with a room-temperature resistivity greater than about 10⁷ohm-centimeter, greater than about 10⁸ ohm-centimeter, greater thanabout 10⁹ ohm-centimeter, greater than about 10¹⁰ ohm-centimeter, orgreater than about 10¹¹ ohm-centimeter. In certain embodiments, thefree-standing ammonothermal group III metal nitride wafer is highlytransparent, with an optical absorption coefficient at a wavelength of400 nanometers that is less than about 10 cm⁻¹, less than about 5 cm⁻¹,less than about 2 cm⁻¹, less than about 1 cm⁻¹, less than about 0.5cm⁻¹, less than about 0.2 cm⁻¹, or less than about 0.1 cm⁻¹.

In some embodiments, the free-standing ammonothermal group III metalnitride crystal or wafer is used as a seed crystal for further bulkgrowth. In one specific embodiment, the further bulk growth comprisesammonothermal bulk crystal growth. In another specific embodiment, thefurther bulk growth comprises high temperature solution crystal growth,also known as flux crystal growth. In yet another specific embodiment,the further bulk growth comprises HVPE. The further-grown crystal may besliced, lapped, polished, etched, and/or chemically-mechanicallypolished into wafers by methods that are known in the art. The surfaceof the wafers may be characterized by a root-mean-square surfaceroughness measured over a 10-micrometer by 10-micrometer area that isless than about 1 nanometer or less than about 0.2 nanometers.

A wafer may be incorporated into a semiconductor structure. Thesemiconductor structure may comprise at least oneAl_(x)In_(y)Ga_((1-x-y))N epitaxial layer, where 0≤x, y, x+y≤1. Theepitaxial layer may be deposited on the wafer, for example, bymetalorganic chemical vapor deposition (MOCVD) or by molecular beamepitaxy (MBE), according to methods that are known in the art. At leasta portion of the semiconductor structure may form a portion of agallium-nitride-based electronic device or optoelectronic device, suchas a light emitting diode, a laser diode, a photodetector, an avalanchephotodiode, a photovoltaic, a solar cell, a cell forphotoelectrochemical splitting of water, a transistor, a rectifier, anda thyristor; one of a transistor, a rectifier, a Schottky rectifier, athyristor, a p-i-n diode, a metal-semiconductor-metal diode,high-electron mobility transistor, a metal semiconductor field effecttransistor, a metal oxide field effect transistor, a power metal oxidesemiconductor field effect transistor, a power metal insulatorsemiconductor field effect transistor, a bipolar junction transistor, ametal insulator field effect transistor, a heterojunction bipolartransistor, a power insulated gate bipolar transistor, a power verticaljunction field effect transistor, a cascode switch, an inner sub-bandemitter, a quantum well infrared photodetector, a quantum dot infraredphotodetector, and combinations thereof. The gallium-nitride-basedelectronic device or optoelectronic device may be incorporated into alamp or a fixture, such as a luminaire. The gallium-nitride-basedelectronic device or optoelectronic device, after singulation, may havelateral dimensions of at least 0.1 millimeter by 0.1 millimeter. Thegallium-nitride-based electronic or optoelectronic device may have amaximum dimension of at least 8 millimeters and, for example, maycomprise a laser diode. The gallium-nitride-based electronic oroptoelectronic device may be entirely free of dislocations throughoutits volume. For example, at a dislocation density of 10⁴ cm⁻², asubstantial fraction of 0.1×0.1 mm² devices could be expected to be freeof dislocations. At a dislocation density of 10² cm⁻², a substantialfraction of 1×1 mm² devices could be expected to be free ofdislocations. The gallium-nitride-based electronic or optoelectronicdevice may be entirely free of stacking faults throughout its volume.For example, at a stacking fault density of 1 cm⁻¹, a substantialfraction of 10×1 mm² stripe-shaped devices, such as laser diodes withnonpolar or semipolar large area surfaces and c-plane facets, could beexpected to be free of stacking faults.

FIGS. 6A, 6B, 7A, and 7B are cross-sectional diagrams illustratingmethods and resulting opto-electronic and electronic devices accordingto embodiments of the present disclosure. A two- or three-terminaldevice, such as an opto-electronic or electronic device, may be formedby a sequence of steps, including the step of epitaxial layer depositionatop a free-standing ammonothermal group III metal nitride wafer 431 orsubstrate having a pattern of locally-approximately-linear arrays 419 ofthreading dislocations and comprising at least one AlInGaN active layer631, e.g., by MOCVD as shown in FIG. 6B. In certain embodiments, thedeposited layers include an n-type or n+ layer 633, a doped orunintentionally doped single quantum well (SQW), a multiple quantum well(MQVV) structure or double-heterostructure (DH structure), or an n-driftlayer, and a p-type layer 635, as shown. The device structures may bevertical, as illustrated schematically in FIG. 6B, or lateral, asillustrated schematically in FIG. 7A. The device may be electricallyconnected to an external circuit to provide a potential between ann-type contact 639 and a p-type contact 637. Additional layers may bedeposited, such as separate confinement heterostructure (SCH) layers,claddings layers, an AlGaN electron-blocking layer, and a p+ contactlayer, among others. In many cases, threading dislocations in thesubstrates, such as coalescence fronts 419, will propagate into thedeposited layers and potentially impact device performance.

In a specific embodiment, the method also deposits an n-type contact639, and a p-type contact 637 as shown in FIGS. 6B and 7A. In someembodiments, at least one of the set of n-type and p-type contacts isplaced in specific registry respect to the coalescence fronts, wingregions, and/or window regions. A light emission portion may be centeredover the coalescence front, or between coalescence fronts. In onespecific embodiment, transparent p-type contacts are deposited and areplaced in such a way that they avoid contact with coalescence fronts,which may have an elevated concentration of threading dislocations. Inthis way a light-emitting structure may be formed has a relatively lowconcentration of threading dislocations. In this way a light-emittingstructure, PN diode, or Schottky barrier diode may be formed has arelatively low concentration of threading dislocations. In preferredembodiments, regions of light emission and/or maximum electric fieldsare designed to overlie wing regions 417 and to avoid coalescence frontregions 419. In certain embodiments, a defective region associated witha coalescence front or a window region is utilized as a shunt path forreducing series resistance. In certain embodiments, n-type contacts areplaced above coalescence fronts or window regions, with an edgedislocation density above 10³ cm⁻¹ and/or a threading dislocationdensity greater than about 10⁵ cm⁻².

Referring now to FIG. 7A, in some embodiments, e.g., a laser diode, a PNdiode, or a Schottky barrier diode, the p-contact may be placed in aregion substantially free of coalescence fronts. In certain embodiments,such as a laser diode, a laser ridge or stripe structure 740 may beplace in a region substantially free of coalescence fronts. A mesa maybe formed by conventional lithography and an n-type contact placed inelectrical contact with the n-type layer and/or the substrate.Additional structures may be placed in registry with the coalescencefronts, such as sidewall passivation, an ion implanted region, fieldplates, and the like.

Referring now to FIG. 7B, in some embodiments, for example, a currentaperture vertical electron transistor (CAVET), an n-drift layer 731 isdeposited over an n+ contact layer 730, which in turn is deposited onfree-standing, merged ammonothermal group III metal nitride wafer 413.P-type layer 735 is formed above n-drift layer 731 with aperture 732.Following regrowth of the balance of n-drift layer 733, an AlGaN 2Delectron gas layer 736 is deposited. Finally, source contacts 737, draincontact 739, dielectric layer 741, and gate contact 743 are deposited.In preferred embodiments, aperture 732 is positioned away fromcoalescence fronts 419. In preferred embodiments, aperture 732 ispositioned away from window regions 415. In preferred embodiments,aperture 732 is positioned over wing regions 417. Other types ofthree-terminal devices, such as trench CAVETs, MOSFETs, and the like arepositioned so that the regions of maximum electric fields are locatedwithin wing regions 417.

FIG. 8 shows a top view (plan view) of a free-standing GaN substrateformed by ammonothermal lateral epitaxial growth using a mask in theform of a two-dimensional array. The GaN layer grew through thetwo-dimensional array of openings in the original mask layer to formwindow regions 415. Coalescence of the GaN layer may form atwo-dimensional grid of the pattern of locally-approximately-lineararrays 419 of threading dislocations.

FIG. 9A shows a top view of a device structure, for example, of LEDs,where transparent p-contacts 970 have been aligned with respect andplaced so as not to be in contact with either the window regions 415 orthe pattern of locally-approximately-linear arrays 419 of threadingdislocations. FIG. 9B shows a top view of an alternative embodiment of adevice structure, for example, of LEDs, where electrical contacts 980are again aligned with respect to window regions 415 and pattern oflocally-approximately-linear arrays 419 of threading dislocations butnow are positioned above pattern of locally-approximately-linear arrays419 of threading dislocations. FIG. 9C shows a top view of analternative embodiment of a device structure, for example, of aflip-chip LED, where n-type electrical contacts 990 are aligned withrespect to window regions 415 and p-type electrical contacts 995 arealigned between window regions 415.

Individual die, for example, light emitting diodes or laser diodes, maybe formed by sawing, cleaving, slicing, singulating, or the like,between adjacent sets of electrical contacts. Referring again to FIG.9A, slicing may be performed along the pattern oflocally-approximately-linear arrays 419 of threading dislocations.Slicing may also be performed through window regions 415. Referring nowto FIG. 9B, in certain embodiments, slicing may be performed throughwindow regions 415 but not along the pattern oflocally-approximately-linear arrays 419 of threading dislocations.Referring again to FIG. 9C, in certain embodiments slicing is performedneither through the seed regions nor along all coalescence fronts.Depending on the arrangement of the one- or two-dimensional array ofseed regions, the singulated die may have three corners, four corners,or six corners.

The methods described herein provide means for fabricating large-areagroup III metal nitride substrates, albeit having some potentiallydefective regions. The methods described herein provide means forfabricating high-performance light emitting diodes and/or laser diodesthat avoid potential issues associated with defective regions in thelarge-area group III metal nitride substrates.

The above sequence of steps provides a method according to an embodimentof the present disclosure. In a specific embodiment, the presentdisclosure provides a method and resulting crystalline material providedby a high pressure apparatus having structured support members. Otheralternatives can also be provided where steps are added, one or moresteps are removed, or one or more steps are provided in a differentsequence without departing from the scope of the claims herein.

EXAMPLES

Embodiments provided by the present disclosure are further illustratedby reference to the following examples. It will be apparent to thoseskilled in the art that many modifications, both to materials, andmethods, may be practiced without departing from the scope of thedisclosure.

Example 1

A c-plane oriented bulk GaN crystal grown by HVPE, approximately 0.3millimeters thick, was provided for use as a substrate 101 forpatterning and ammonothermal crystal growth. A 100-nanometer-thick layerof TiW was sputter-deposited as an adhesion layer on the (000-1) N-faceof the substrate, followed by a 780-nanometer-thick inert layercomprising Au. A 6-micrometer-thick Au layer was then electroplated onthe sputtered layer, increasing the thickness of the inert layer (e.g.,blanket mask 116). Using AZ-4300 as a photoresist (e.g., photoresistlayer 103), a pattern comprising linear arrays of 3-micrometer-wide by1-centimeter-long slits (e.g., openings 112), with a pitch diameter of1200 micrometers was defined. A wet-etch process was performed, using acommercial TFA gold etching solution at room temperature, as shownschematically in FIGS. 1M-1P, to obtain a substrate with patterned masklayer 111. The mask pattern comprised domains of m-stripes, with linearopenings oriented approximately 30-40 micrometers wide and parallel to<10-10>. The substrate with patterned mask layer 111 was then placed ina stirred beaker with concentrated H₃PO₄. The beaker was heated toapproximately 280 degrees Celsius over approximately 30 minutes, held atthis temperature for approximately 90 minutes, and cooled. A crosssection of a trench 115 formed by this procedure, having a depth ofapproximately 162 micrometers and a width at the top of approximately105 micrometers, is shown in FIG. 10. The sidewalls of the trench 115,remarkably, are nearly vertical.

Example 2

A patterned, trenched c-plane-oriented bulk GaN substrate 101 wasprepared by a similar procedure as that described in Example 1. Thepatterned substrate was placed in a silver capsule along with a15%-open-area baffle, polycrystalline GaN nutrient, NH₄F mineralizer,and ammonia, and the capsule was sealed. The ratios of GaN nutrient andNH₄F mineralizer to ammonia were approximately 1.69 and 0.099respectively, by weight. The capsule was placed in an internally-heatedhigh pressure apparatus and heated to temperatures of approximately 666degrees Celsius for the upper, nutrient zone and approximately 681degrees Celsius for the lower, crystal growth zone, maintained at thesetemperatures for approximately 215 hours, and then cooled and removed.Ammonothermal GaN filled in most of the volume in the trenches, grewthrough the linear openings in the patterned mask on the HVPE GaNsubstrate, grew laterally, and coalesced fully, forming an ammonothermalGaN layer approximately 1200 micrometers thick with a smooth topsurface. Two parallel cuts were made in the ammonothermal GaN layer,perpendicular to both the surface and the patterns, resulting in abar-shaped test specimen with m-plane surfaces. One m-plane surface ofthe test specimen was polished and examined by optical microscopy, asshown in FIGS. 11A and 11B. An interface is visible between thesubstrate 101 and laterally-grown group III metal nitride material 221,as illustrated by the dotted line in the expanded view on the right sideof FIG. 11B. Patterned mask layer 111 and void 225 both appear as blackin the images and underlie ammonothermal group III metal nitride layer213.

Example 3

A patterned, trenched c-plane-oriented bulk GaN substrate was preparedby a similar procedure as that described in Examples 1 and 2, and thefinal group III metal nitride layer 213 is shown in FIG. 12B (i.e.,right side figures). A second patterned substrate was prepared by asimilar procedure except that no trenches were prepared below the maskopenings, and the final group III metal nitride layer is shown in FIG.12A (i.e., left side figures). The patterned substrates were placed in asilver capsule along with a 15%-open-area baffle, polycrystalline GaNnutrient, NH₄F mineralizer, and ammonia, and the capsule was sealed. Theratios of GaN nutrient and NH₄F mineralizer to ammonia wereapproximately 2.05 and 0.099 respectively, by weight. The capsule wasplaced in an internally-heated high pressure apparatus and heated totemperatures of approximately 666 degrees Celsius for the upper,nutrient zone and approximately 678 degrees Celsius for the lower,crystal growth zone, maintained at these temperatures for approximately427 hours, and then cooled and removed. Ammonothermal GaN filled in mostof the volume in the trenches of the trenched substrate (FIG. 12B), grewthrough the linear openings in the patterned mask on the HVPE GaNsubstrate, grew laterally, and coalesced fully, forming an ammonothermalGaN layer approximately 2100 micrometers thick with a smooth topsurface. Ammonothermal GaN layer similarly grew through the linearopenings in the patterned mask on the patterned, untrenched HVPE GaNsubstrate (FIG. 12A), grew laterally, and coalesced fully, forming anammonothermal GaN layer approximately 2100 micrometers thick with asmooth top surface. The surface of both ammonothermal GaN layers werelightly etched and were examined by optical microscopy. Differentialinterference contrast (Nomaraki) micrographs and transmissionmicrographs of both layers are shown in FIGS. 12A-12B. The average etchpit density, which is believed to accurately represent the threadingdislocation density, of the ammonothermal GaN layer grown on thepatterned substrate without trenches (FIG. 12A), was approximately1.0×10⁵ cm⁻². The average etch pit density of the ammonothermal GaNlayer grown on the patterned, trenched substrate (FIG. 12B), wasapproximately 1.0×10⁴ cm⁻², a full order-of-magnitude improvement.

Example 4

A patterned, trenched c-plane-oriented bulk GaN substrate was preparedby a similar procedure as that described in Examples 1 and 2 but with apitch of 800 micrometers. The patterned, trenched substrate was placedin a silver capsule along with a 15%-open-area baffle, polycrystallineGaN nutrient, NH₄F mineralizer, and ammonia, and the capsule was sealed.The ratios of GaN nutrient and NH₄F mineralizer to ammonia wereapproximately 1.71 and 0.099 respectively, by weight. The capsule wasplaced in an internally-heated high pressure apparatus and heated totemperatures of approximately 668 degrees Celsius for the upper,nutrient zone and approximately 678 degrees Celsius for the lower,crystal growth zone, maintained at these temperatures for approximately485 hours, and then cooled and removed. Ammonothermal GaN filled in mostof the volume in the trenches of the trenched substrate, grew throughthe linear openings in the patterned mask on the HVPE GaN substrate,grew laterally, and coalesced fully, forming an ammonothermal GaN layerapproximately 980 micrometers thick with a smooth top surface. The HVPEGaN substrate was removed by grinding, and the resulting free-standingammonothermal GaN substrate was polished and chemical-mechanicalpolished. The free-standing ammonothermal GaN substrate was thencharacterized by x-ray diffraction, using a PANalytical X'Pert PROdiffractometer using an electron energy of 45 kV with a 40 mA linefocus, a 0.0002 degree step, a 1 sec dwell time, an Ge(220) mirror, aslit height of 1.0 mm and a slit width of 1.0 mm, at nine differentlocations across the substrate. The results of an analysis of the formedGaN substrate are summarized in FIG. 13. The range of miscut along[1-100] was measured to be 0.078 degrees over the central 80% of thelarge area surface of the crystal, and the range of miscut along [11-20]was measured to be 0.063 degrees over the central 80% of the large areasurface of the crystal. Thus, in some embodiments, a free standingcrystal having a miscut angle that varies by 0.1 degree or less in thecentral 80% of the large area surface of the crystal along a firstdirection and a miscut angle that varies by 0.1 degree or less in thecentral 80% of the large area surface of the crystal along a seconddirection orthogonal to the first direction. By contrast, an identicalmeasurement on a commercial HVPE wafer resulted in a range of miscutalong [1-100] of 0.224 degrees and a range of miscut along [11-20] of0.236 degrees. The full width at half maximum of the rocking-curve forthe (002) reflection was measured as 36 arc seconds, while that of the(201) reflection was measured as 32 arc seconds, as summarized in thetables and graphs shown FIG. 14. By contrast, identical measurements ona 50 mm diameter, commercial HVPE substrate produced values of 48 and 53arc seconds, respectively, and identical measurements on a 100 mmdiameter, commercial HVPE substrate produced values of 78 and 93 arcseconds, respectively.

Example 5

A c-plane oriented bulk GaN crystal grown by HVPE, approximately 0.3millimeters thick, was provided for use as a substrate for patterningand ammonothermal crystal growth. A 100-nanometer-thick layer of TiW wassputter-deposited as an adhesion layer on the (000-1) N-face of thesubstrate, followed by a 780-nanometer-thick inert layer comprising Au.A 6-micrometer-thick Au layer was then electroplated on the sputteredlayer, increasing the thickness of the inert layer. A pattern was formedon the N-face of the substrate using a frequency-doubled YAG laser withnano-second pulses. The pattern comprised domains of m-trenches, withlinear openings oriented approximately 50-60 micrometers wide andparallel to <10-10>, with a pitch of 1200 micrometers. The patternedsubstrate was then placed in a stirred beaker with concentrated H₃PO₄.The beaker was heated to approximately 280 degrees Celsius overapproximately 30 minutes, held at this temperature for approximately 60minutes, and cooled. A cross section of a trench formed by thisprocedure, having a depth of approximately 200 micrometers and a widthat the top of approximately 80 micrometers, is shown in FIG. 15. Thesidewalls of the trench, remarkably, are nearly vertical.

Example 6

A patterned, trenched c-plane-oriented bulk GaN substrate was preparedby a similar procedure as that described in Example 5, except that ahigher power was used for the laser so that slots were formed that fullypenetrated the substrate. After etching with concentrated H₃PO₄ atapproximately 280 degrees Celsius for approximately 30 minutes, thewidth of the slots was approximately 115 micrometers. The patternedsubstrates were placed in a silver capsule along with a 15%-open-areabaffle, polycrystalline GaN nutrient, NH₄F mineralizer, and ammonia, andthe capsule was sealed. The ratios of GaN nutrient and NH₄F mineralizerto ammonia were approximately 1.74 and 0.099 respectively, by weight.The capsule was placed in an internally-heated high pressure apparatusand heated to temperatures of approximately 667 degrees Celsius for theupper, nutrient zone and approximately 681 degrees Celsius for thelower, crystal growth zone, maintained at these temperatures forapproximately 500 hours, and then cooled and removed. Ammonothermal GaNfilled in most of the volume in the trenches of the trenched substrate,grew through the linear openings in the patterned mask on the HVPE GaNsubstrate, grew laterally, and coalesced fully, forming an ammonothermalGaN layer approximately 2010 micrometers thick with a smooth topsurface. The surface of the ammonothermal GaN layer was lightly etchedand was examined by optical microscopy. An optical micrograph of thelayer is shown in FIG. 16. The etch pits in the rectangles A, B, C, D,E, F, and G shown in FIG. 16 were counted, leading to a determinationthat the average etch pit density, which is believed to accuratelyrepresent the threading dislocation density, of the ammonothermal GaNlayer grown on the patterned, laser-trenched substrate, wasapproximately 6.0×10³ cm⁻².

While the foregoing is directed to embodiments of the presentdisclosure, other and further embodiments of the disclosure may bedevised without departing from the basic scope thereof, and the scopethereof is determined by the claims that follow.

What is claimed is:
 1. A free-standing crystal, comprising a group Illmetal and nitrogen, wherein the free-standing crystal comprises: awurtzite crystal structure; a first surface having a maximum dimensiongreater than 40 millimeters in a first direction; an averageconcentration of stacking faults below 10³ cm⁻¹, an averageconcentration of threading dislocations between 10¹ cm⁻² and 10⁶ cm⁻²,wherein the average concentration of threading dislocations on the firstsurface varies periodically by at least a factor of two in the firstdirection, the period of the variation in the first direction beingbetween 5 micrometers and 20 millimeters; and a miscut angle that variesby 0.1 degree or less in the central 80% of the first surface of thecrystal along the first direction and by 0.1 degree or less in thecentral 80% of the first surface of the crystal along a second directionorthogonal to the first direction, wherein the first surface comprises aplurality of first regions, each of the plurality of first regionshaving a locally-approximately-linear array of threading dislocationswith a concentration between 5 cm⁻¹ and 10⁵ cm⁻¹, the first surfacefurther comprises a plurality of second regions, each of the pluralityof second regions being disposed between an adjacent pair of theplurality of first regions and having a concentration of threadingdislocations below 10⁵ cm⁻² and a concentration of stacking faults below10³ cm⁻¹, and the first surface further comprises a plurality of thirdregions, each of the plurality of third regions being disposed withinone of the plurality of second regions or between an adjacent pair ofsecond and having a minimum dimension between 10 micrometers and 500micrometers and threading dislocations with a concentration between 10³cm⁻² and 10⁶ cm⁻².
 2. The crystal of claim 1, wherein the crystalfurther comprises an impurity concentration of H greater than 10¹⁷ cm⁻³,and an impurity concentration of at least one of Li, Na, K, F, Cl, Br,and I greater than 10¹⁵ cm⁻³, as quantified by calibrated secondary ionmass spectrometry.
 3. The crystal of claim 2, wherein the first surfaceis characterized by impurity concentrations of oxygen (O), hydrogen (H),and at least one of fluorine (F) and chlorine (Cl) between 1×10¹⁶ cm⁻³and 1×10¹⁹ cm⁻³, between 1×10¹⁶ cm⁻³ and 2×10¹⁹ cm⁻³, and between 1×10¹⁵cm⁻³ and 1×10¹⁹ cm⁻³, respectively.
 4. The crystal of claim 2, whereinthe first surface is characterized by impurity concentrations of oxygen(O), hydrogen (H), and at least one of sodium (Na) and potassium (K)between 1×10¹⁶ cm⁻³ and 1×10¹⁹ cm⁻³, between 1×10¹⁶ cm⁻³ and 2×10¹⁹cm⁻³, and between 3×10¹⁵ cm⁻³ and 1×10¹⁸ cm⁻³, respectively.
 5. Thecrystal of claim 2, wherein a ratio of the impurity concentration of Hto an impurity concentration of 0 is between about 0.3 and about
 100. 6.The crystal of claim 1, wherein the first surface is characterized by asymmetric x-ray rocking curve full width at half maximum less than 50arcsec, as measured by an x-ray diffractometer with a slit height of 1millimeter.
 7. The crystal of claim 1, wherein a pitch dimension betweenadjacent pairs of first regions is between 400 micrometers and 5millimeters.
 8. The crystal of claim 1, wherein the first surface ischaracterized by a crystallographic orientation within 5 degrees of the{10-10} m-plane.
 9. The crystal of claim 1, wherein the first surface ischaracterized by a crystallographic orientation within 5 degrees of the(0001)+c-plane or within 5 degrees of the (000-1)−c-plane.
 10. Thecrystal of claim 1, wherein the first surface is characterized by acrystallographic orientation within 5 degrees of a semipolar orientationselected from {6 0−6±1}, {5 0−5±1}, {4 0−4±1}, {3 0−3±1}, {5 0−5±2}, {70−7±3}, {2 0−2±1}, {3 0−3±2}, {4 0−4±3}, {5 0−5±4}, {1 0−1±1}, {10−1±2}, {1 0−1±3}, {2 1−3±1}, and {3 0−3±4}.
 11. The crystal of claim 1,wherein the locally-approximately-linear arrays of the plurality offirst regions are oriented within 5 degrees of a crystallographic planeselected from <1 0−1 0>, <1 1−2 0>, and [0 0 0±1], and a projection ofthe crystallographic plane on the first surface.
 12. The crystal ofclaim 1, further comprising a second surface, wherein the second surfaceis substantially parallel to the first surface; and the crystal ischaracterized by a thickness between the first surface and the secondsurface between about 0.1 millimeter and about 1 millimeter, by a totalthickness variation of less than about 10 micrometers, and by amacroscopic bow less than about 50 micrometers.
 13. A method for forminga gallium-containing nitride crystal, comprising: depositing a patternedmask layer on a first surface of a substrate, wherein the substrate isselected from one of single-crystalline group-III metal nitride,gallium-containing nitride, and gallium nitride, and has a concentrationof threading dislocations less than 10⁸ cm⁻² and a concentration ofstacking faults less than 10⁴ cm⁻¹, and the pattered mask layercomprises an array of openings that have a pitch in a first directionbetween 5 micrometers and 20 millimeters; removing portions of thesubstrate exposed within the array of openings to form trenches in thesubstrate, the trenches having a depth below the first surface ofgreater than 50 micrometers; and performing a bulk crystal growthprocess using the substrate as a seed crystal.
 14. The method of claim13, wherein the trenches have a width between 10 micrometers and 200micrometers and a length between 100 micrometers and 50 millimeters. 15.The method of claim 13, wherein the patterned mask layer comprises aninert layer overlying an adhesion layer, wherein the adhesion layercomprises one or more of Ti, TiN, TiN_(y), TiSi₂, Ta, TaN_(y), Al, Ge,Al_(x)Ge_(y), Cu, Si, Cr, V, Ni, W, TiW_(x), TiW_(x)N_(y) and has athickness between 1 nanometer and 1 micrometer and the inert layercomprises one or more of Au, Ag, Pt, Pd, Rh, Ru, Ir, Ni, Cr, V, Ti, orTa and has a thickness between 10 nanometers and 100 micrometers. 16.The method of claim 15, wherein the pattered mask layer furthercomprises a diffusion barrier layer between the adhesion layer and theinert layer, the diffusion barrier layer comprising one or more of TiN,TiN_(y), TiSi₂, W, TiW_(x), TiN_(y), WN_(y), TaN_(y), TiW_(x)N_(y),TiW_(x)Si_(z)N_(y), TiC, TiCN, Pd, Rh, or Cr, and having a thicknessbetween 1 nanometer and 10 micrometers.
 17. The method of claim 13,wherein the bulk crystal growth process comprises: placing the substratein a sealable container along with a group III metal source, amineralizer and ammonia; heating the sealable container to a temperatureabove 400 degrees Celsius, thereby causing an ammonothermal group IIImetal nitride material to grow within the trenches, within the array ofopenings, outward through the array of openings, and subsequentlylaterally over the patterned mask layer; and pressurizing it to apressure above 50 MPa for a duration of at least 100 hours.
 18. Themethod of claim 13, wherein the bulk crystal growth process causes agroup III metal nitride material to grow laterally over the patternedmask layer and coalesce to form one or more coalescence fronts, whereinthe one or more coalescence fronts comprise a pattern oflocally-approximately-linear arrays of threading dislocations that havea concentration between 5 cm⁻¹ and 10⁵ cm⁻¹.
 19. The method of claim 13,wherein the openings in the patterned mask layer are formed by alithography process and the trenches are formed by a wet etchingprocess.
 20. The method of claim 13, wherein the openings in thepatterned mask layer and the trenches are formed by a laser process. 21.The method of claim 20, wherein the laser process comprises scanningover individual locations repetitively to form openings in the patternedmask layer and trenches having predetermined dimensions.
 22. The methodof claim 21, wherein the laser process comprises scanning over an entirepattern on the first surface repetitively to form openings in thepatterned mask layer and trenches having predetermined dimensions. 23.The method of claim 20, wherein residual damage in the trenches isremoved by a wet etching or by a photoelectrochemical etching process.24. The method of claim 13, wherein the substrate comprisessingle-crystal gallium nitride, the first surface has a crystallographicorientation within about 5 degrees of (000-1) −c-plane, and the trenchesare formed by an etching process comprising H₃PO₄.
 25. The method ofclaim 13, wherein each of the openings of the pattered mask layer has ashape selected from round, square, rectangular, triangular, andhexagonal, and has a size between 1 micrometer and 5 millimeters.